1. Field of the Invention
This invention relates to a technology for detecting misfire occurring during operation of a gasoline or other spark-ignition internal combustion engine and more particularly to a spark plug cap with a capacitor for detecting ignition voltage in order to detect such a misfire, and yet more particularly to a spark plug cap having such capacitor and being further provided with an ignition coil which is most suitable for the so-called distributorless ignition system.
2. Description of the Prior Art
As is well known, in gasoline and other types of internal combustion engines a high voltage produced by an ignition coil is distributed to spark plugs at the engine cylinders by an ignition distributor. At each cylinder, the resulting electric discharge between the spark plug electrodes produces a spark which ignites an air-fuel mixture that has been drawn into the cylinder and compressed at the proper time, causing the mixture to burn explosively. In the course of this ignition-combustion process in the internal combustion engine, the mixture may for some reason occasionally fail to burn properly. This is referred to as a misfire. Misfires can result from causes in either the fuel supply system or the ignition system. Misfires caused by problems in the fuel supply system are the result of an overly lean or overly rich air-fuel mixture. At that instance, a spark is produced between the spark plug electrodes but the air-fuel mixture does not ignite. Misfires caused by problems in the ignition systems are the result of spark plug electrodes fouling or ignition circuit malfunctions which prevent normal spark discharge.
The occurrence of misfire in the course of engine operation not only degrades engine performance but may also degrade fuel efficiency. It may further cause after-firing of unburned gases in the exhaust system, which can affect the exhaust emission control system and have other adverse effects. Moreover, since the occurrence of even a single misfire indicates a misadjustment or malfunction in the fuel supply system or ignition system, prompt elimination of the problem is essential. Because of this, there is a strong need for development of a detector for detecting misfires as soon as they occur.
One type of misfire detector that has been proposed is the mis-spark detector described in Japanese Laid-open Patent Publication No. 52(1977)-118135. As shown in FIG. 6, the detector includes a conductor 51 wrapped around a portion of a high-voltage (high tension) cable 50 of the engine ignition system so as to constitute a detective capacitor 52 (a type of capacitance probe) in which the insulation cladding 50A of the high-voltage cable 50 serves as the dielectric. A voltage divider capacitor 53 is connected between the capacitor 52 and the ground so that the ignition voltage (secondary voltage of the ignition coil) applied to the conductive core 50B of the high-voltage cable 50 induces a voltage across the terminals of the capacitor 52 owing to its static capacitance. The induced voltage is statically divided by the capacitor 52 and the capacitor 53, and the voltage across the terminals of the capacitor 53 (the divided voltage) is forwarded as a detection voltage to an electronic circuit 54 for processing and discrimination. The electronic circuit 54 discriminates the occurrence of misfires from the difference between the wave form of the ignition voltage at the time of normal spark discharge and that at the time of no spark discharge (mis-sparking). Among the different types of misfires, the detector thus detects misfires that occur when no spark discharge is produced owing to a problem in the ignition system.
Another detector for detecting misfire in internal combustion engines is disclosed in the present assignee's Japanese Laid-Open Patent Publication No. 5(1993)-65868. In this detector, the ignition voltage is similarly detected from a high-voltage cable or the like of the ignition system using static voltage division, and misfiring owing to causes in the fuel supply system is detected based on the fact that, even when spark discharge occurs, the wave form of the ignition voltage differs between the case where normal combustion occurs and the case where it does not.
In conventional misfire detectors, such a "capacitive probe" is used as a means for detecting ignition voltage. The probe is constituted by wrapping a sheet or ribbon of conductor around the high-voltage cable of the ignition system so as to form a detective capacitor between the conductor and the core of the high-voltage cable using the insulation cladding of the high-voltage cable as the dielectric. However, the capacitance probe constituted in this manner has a major drawback that derives from the nature of the high-voltage cable of the ignition system. Because of its flexibility and elasticity, the high-voltage cable is highly susceptible to vibration. It is also easily affected by changing ambient humidity, wetting by leaking water, and fouling with oil, grime and the like. When a capacitor for use in detection is formed by wrapping a conductor ribbon around the cable, the static capacitance of the capacitor is apt to be changed from its proper value by a shifting of the conductor caused by vibration as well as by changing humidity, wetting with water and fouling with oil, grime and the like. Although some change in static capacitance can be tolerated if the capacitor is to be used only for checking the ignition voltage, even slight changes have to be avoided when it is used for misfire detection, because such detection generally requires accurate detection not only of the ignition voltage but also of the ignition voltage wave form. The capacitance changes to which the prior art capacitance probe is susceptible may alter the detected voltage wave form and thereby make it impossible to detect misfire with high reliability.
In actual practice, moreover, the flexibility and elasticity of the high-voltage cable make the work of attaching the conductor for forming the capacitor on the insulation cladding of the high-voltage cable and securing it thereon difficult and troublesome. Maintenance of the so-formed capacitor is also troublesome.
In addition, the insulation cladding of the high-voltage cable is generally formed of synthetic rubber, a material that is readily degraded when exposed to heat and/or fouled with oil and grime. This degradation of the insulation cladding after the detection capacitor has been formed by winding the conductor around the high-voltage cable not only produces a progressive change in the static capacitance of the capacitor over time, but may also reduce the electric insulation property of the cladding to the point that the high ignition voltage can leak to the conductor wrapped around it. When this happens, the high leak voltage is apt to be conducted to the electronic circuitry of the misfire detector, which it can damage or cause to malfunction. Ideally, moreover, for enabling accurate detection of the ignition voltage wave form, the distance between the conductor and the conductive core constituting the opposite sides of the detective capacitor should be made as small as possible so as to increase its static capacitance. In actuality, however, the conductor and the cable conductive core have to be spaced at a greater distance than desirable as a precaution against loss of the insulation property in the cable cladding owing to the aforesaid causes as well as to corona discharge. As a result, it has been impossible to avoid low accuracy in detecting the ignition voltage wave form.
In order to solve the problems, the present assignee proposed a spark plug cap with voltage detective capacitor for an internal combustion engine, in which a conductor for forming the ignition voltage detective capacitor is provided in a spark plug's cap, all within in the secondary ignition circuit (Japanese Patent Application No. 4(1992)-56395; also U.S. Pat. No. 5,363,046 issued Nov. 8, 1995. More specifically, in a spark plug cap covering the terminal of a spark plug and having a conductor for carrying ignition voltage current from the ignition coil to the spark plug terminal to generate an electronic spark between spark plug electrodes and having an insulator surrounding this conductor, a second conductor is disposed in the insulator around the first conductor maintaining a predetermined distance therefrom so as to form a detective capacitor between the first and the second conductors.
With this arrangement, since the spark plug cap is fixed on the spark plug so as to cover its terminal portion and the spark plug itself is screwed firmly into the head of the internal combustion engine and since the conductor for detection is formed integrally with the spark plug cap insulator, the thus-formed detective capacitor is not only improved in its structure and hence in its service life, but in addition any vibration it does receive will neither shift its position nor deform it. In addition, since the secondary conductor is located inside the spark plug cap, the detective capacitor is isolated from changes in humidity and other ambient conditions and is also protected from the invasion of oil, grime and the like from the outside. As a result, the ignition voltage wave form can always be accurately detected, thereby ensuring accurate discrimination of misfire occurrence. Furthermore, since the conductor is disposed integrally in the cap's insulator at the time of molding it, this eliminates the tedious attaching work and the troublesome maintenance required before.
Aside from the issues raised above, the last several years have seen increasing use of the so-called distributorless or direct ignition system, in which each spark plug is provided with its own ignition coil, from which it receives secondary ignition voltage directly. In this system, the ignition coil is often attached integrally to the spark plug cap at its top. And in such a spark plug cap with the ignition coil, the cap's insulator body for surrounding the conductor carrying high voltage current to the spark plug is usually mold-formed from a material of hard and rigid resin. Since the detective conductor can be provided in the hard and rigid insulator body such that it is firmly secured in position, and therefore ensures accurate detection of the ignition voltage, a spark plug cap of this kind is most suitable as a housing for the voltage detective capacitor under discussion.
When introducing the detective capacitance under discussion to such a spark plug cap as houses an ignition coil, however, a difficulty arises in actually manufacturing the same. More specifically, the detective conductor constituting the ignition voltage detective capacitor should have conductors such as wires or cables for extracting the detected voltage to the exterior and forwarding it to a signal processing circuit or the like. In most vehicles, the place where the spark plug cap is positioned is so narrow that it is often difficult to locate conductors including those for the detective conductor. Since use of the spark plug cap under discussion would increase the number of detectors or cords over the number used with an ordinary cap, assembling such a spark plug cap in a predetermined location becomes more tedious and time-consuming, thereby decreasing working efficiency.